The EIA RS-170A tentative standard (1977) for NTSC-based television systems sets the period of the horizontal synchronization pulse train at 227.5 times that of the color subcarrier signal. Since an integer relationship does not exist between the periods of these signals, the phase angle between the color subcarrier and horizontal synchronization pulse train shifts by 180.degree. for each successive horizontal scan line. Moreover, since 525 horizontal scan lines define a color frame (i.e., two color fields), the phase angle between the color subcarrier and the horizontal synchronization pulse train also shifts by 180.degree. for each successive color frame. The NTSC-based color television standard establishes, therefore, a two-color frame (i.e., four-color field) sequence of phase angles between the color subcarrier and the horizontal synchronization pulse train.
The editing of a single video signal or the multiplexing of multiple asynchronous video signals requires the synchronization or matching of the phase angle sequences of the components of the resulting composite video signal to provide a continuous series of video images with smooth transitions between them. The absence of matching of the phase angle sequences in, for example, a video tape recorder (VTR) can create problems that manifest themselves in the following ways. Whenever there is a mismatch between the color frame sequence of a color video signal being edited and the timing sequence of the color video reference signal in the VTR, the recording of the color video signal could be delayed or might not take place at all. Whenever there is a phase difference between successive color subcarrier signals of the color video signals provided by separate signal sources during a multiplexing operation, there could occur a momentary shift in color at the time the signals are switched. Whenever the phases of the color subcarrier signals are the same but the color frame sequences are mismatched, there could occur a horizontal jump in the picture at the time the signals are switched.
The maintenance of the proper phase angle between the color subcarrier and horizontal synchronization pulse train of a color video reference signal is important to the operation of a television studio. A phase angle offset could cause similar offsets in equipment receiving the reference signal, thereby resulting in any one of the video signal processing errors described hereinabove. The phase and frequency of the color subcarrier signal are the same as those of the color synchronization burst signal, which is located on the back porch of a horizontal blanking pulse. The phase angle between the color subcarrier and the horizontal synchronization pulse can be determined, therefore, by measuring with a phase detector the phase angle between the color synchronization burst signal and the horizontal synchronization pulse.
A phase detector of conventional design employs a phase-locked loop (PLL), which phase locks the output signal of a voltage-controlled oscillator to the horizontal synchronization pulse. A determination of the phase relationship between the color synchronization burst and voltage-controlled oscillator output signal provides an indication of the phase angle between the color synchronization burst and the horizontal synchronization pulse. Since it must operate in the presence of jitter or phase noise on the color video signal, a PLL is typically designed to have a relatively large acquisition time that would make difficult the phase measurement of horizontal synchronization pulses in consecutive scan lines.